Highnote is an excellent little app for iOS that allows you to change the tempo and key of the music in your library.
Like all Mac and iOS apps that require access to your device’s music library, it can only work with audio files that are locally downloaded. My iTunes Match subscription allows me to stream my iTunes library from multiple Macs and iDevices without the files occupying any space. Other apps cannot access songs in the cloud unless they are also downloaded to the device. This is a disappointment of mine that I wish Apple would address. While it is a drag to have to go into the Music app and download songs before use, Highnote is still a great little utility for slowing down, speeding up, or changing the key of the songs in your music library, tasks I typically associate with doing on a desktop computer.

For $1.99, it is worth checking out. And they’ve already announced an Apple Watch app! Good for them, though I’m not sure I will be needing to use these tools on my wrist.